Magnetic resonance probes

ABSTRACT

A magnetic resonance probe may include a plurality of center conductors, at least some center conductors including a conductive core and an insulator disposed at least partially about the core along at least a portion of the core, a first dielectric layer disposed at least partially about the plurality of center conductors in a proximal portion of the probe, an outer conductive layer at least partially disposed about the first dielectric layer, and a plurality of electrodes, at least one electrode being coupled to one of the center conductors and disposed at least partly on a probe surface.

Notice: More than one reissue application has been filed for the reissueof U.S. Pat. No. 6,904,307. The reissue applications are U.S.application Ser. No. 11/810,679 filed Jun. 6, 2007 which issued on Oct.18, 2011 as U.S. Pat. No. Re. 42,856 E, and the present application(U.S. application Ser. No. 13/027,489), which is a divisional reissue ofU.S application Ser. No. 11/810,679.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S provisional application Ser.No 60/383,828, filed May 29, 2002, which is hereby incorporated hereinin its entirety by this reference.

BACKGROUND

Leads (catheters) for a wide variety of medical procedures, such as DeepBrain Stimulation (DBS) and cardiac interventions, are typically placedinto the body of a subject under stereotactic guidance, fluoroscopy, orother methods. Stereotactic guidance is a static method based on highresolution images taken prior to the procedure and does not take intoaccount displacement of the brain caused by the loss of cerebral spinalfluid (CSF), blood or simple brain tissue displacement by the surgicaltool. It is therefore often necessary to perform a real timephysiological localization of the target area to augment and verify thepreviously obtained stereotactic data by observing the patients responseto stimulation through the DBS electrodes or by recording and displaying(visual or audible) the action potentials of individual neurons alongthe path way to the target zone using microelectrodes. These additionalsteps are time consuming; resulting in procedures between 6-8 hours witha failure rate still remaining between 20-30%.

Cardiac procedures are mainly performed using X-ray fluoroscopy. BecauseX-ray shadows are the superposition of contributions from manystructures, and since the discrimination of different soft tissues isnot great, it is often very difficult to determine exactly where thecatheter is within the heart. In addition, the borders of the heart aregenerally not accurately defined, so it is generally not possible toknow whether the catheter has penetrated the wall of the heart.Furthermore, lesions are invisible under x-ray fluoroscopy. Thus, it isvery difficult to discern whether tissue has been adequately ablated.

SUMMARY

The systems and methods disclosed herein may simplify the manufacturingprocess for magnetic resonance probes, increase patient safety, reduceif not eliminate tissue heating, and facilitate the performance ofmultiple functions during MRI interventional procedures such as DeepBrain Stimulation, Electrophysiological Mapping, and/or RF Ablation.

In an embodiment, a magnetic resonance probe may include a plurality ofcenter conductors, at least some center conductors including aconductive core and an insulator disposed at least partially about thecore along at least a portion of the core. A first dielectric layer maybe disposed at least partially about the plurality of center conductorsin a proximal portion of the probe. An outer conductive layer may be atleast partially disposed about the first dielectric layer. A pluralityof electrodes may be included, at least one electrode being coupled toone of the center conductors and disposed at least partly on a probesurface.

In an embodiment, a probe may include a second dielectric layer at leastpartially disposed about the outer conductor. In an embodiment, theplurality of center conductors may be magnetic resonance-compatible. Inan embodiment, at least one insulator may have a thickness up to about100 microns. In an embodiment, at least some center conductors may forma first pole of a dipole antenna, and the outer conductive layer mayform a second pole of the dipole antenna. In an embodiment, a probe caninclude a plurality of radially expandable arms. In an embodiment, atleast one electrode may be at least partly disposed on an arm.

In an embodiment, an interface circuit may be electrically coupled tothe probe, the interface circuit including a signal splitter thatdirects a signal received from the probe to a magnetic resonance pathwayand an electrophysiology pathway, a high-pass filter disposed in themagnetic resonance pathway, a low-pass filter disposed in theelectrophysiology pathway, a connector disposed in the magneticresonance pathway for connecting to a magnetic resonance scanner, and aconnector disposed in the electrophysiology pathway for connecting to atleast one of a tissue stimulator, a biopotential recording system, andan ablation energy source.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosed systems and methods will be apparent fromthe following more particular description of exemplary embodiments asillustrated in the accompanying drawings, in which some referencecharacters refer to the same parts throughout the various views. Thedrawings are not necessarily to scale, nor are individual elementsnecessarily in relative proportion to other elements, emphasis insteadbeing placed upon illustrating principles of the disclosed systems andmethods.

FIG. 1 depicts an exemplary embodiment of a magnetic resonance probehaving four center conductors and four electrodes.

FIGS. 2A-C depict an exemplary embodiment of a magnetic resonance probehaving four center conductors and four electrodes. FIG. 2A depicts aside view. FIG. 2B depicts a cross section in a distal portion of theprobe. FIG. 2C depicts a cross section in a proximal portion of theprobe.

FIGS. 3A-E depict exemplary embodiments of an interface circuit. FIGS.3A and 3B depict exemplary electrical schematics; FIGS. 3C-3E depictexemplary physical layouts.

FIGS. 4A-4C depict an exemplary embodiment of a steerable magneticresonance probe.

FIGS. 5A-5C depict an exemplary embodiment of a magnetic resonance probehaving cooling lumens.

FIGS. 6A-6D depicts an exemplary embodiment of a magnetic resonanceprobe having expandable arms. FIG. 6A depicts a side view of theexemplary probe. FIG. 6B depicts a long axis view of an arm. FIG. 6Cdepicts a cross section of expanded arms. FIG. 6D depicts a crosssection in a proximal portion of the exemplary probe.

FIGS. 7A-B show heating profiles of tissue surrounding an exemplarymagnetic resonance probe in the transmit mode that is decoupled (FIG.7A) or not decoupled (FIG. 7B).

FIGS. 8A-C depict an exemplary embodiment of a bidirectionally steerablemagnetic resonance probe having wires that are both pull wires andcenter conductors.

FIGS. 9A-C depict an exemplary embodiment of a unidirectionallysteerable magnetic resonance probe having an offset wire that is both apull wire and center conductor.

FIGS. 10A-C depict an exemplary embodiment of a unidirectionallysteerable magnetic resonance probe having a centered wire that is both apull wire and center conductor.

DETAILED DESCRIPTION

The disclosed systems and methods relate to the guidance andvisualization of diagnostic and therapeutic procedures performed underMagnetic Resonance Imaging (MRI). Such procedures in general benefitfrom the excellent soft tissue contrast obtainable with MRI. Examples ofsuch applications are Deep Brain Stimulation (DBS) for the treatment ofmovement disorders (Parkinson's disease, Essential tremor, etc.) andother neurological disorders benefiting from electrical stimulations ofsection of the brain, as well as the diagnosis and treatment of cardiacarrhythmias including but not limited to atrial fibrillation andventricular tachycardia.

Real time Magnetic Resonance Imaging can overcome both the inaccuraciesof stereotactic planning and the lack of soft tissue contrast as foundin X-ray fluoroscopy. The use of Magnetic Resonance Imaging guidedinterventions can therefore result in shortened procedure times andincreased success rates.

Some conditions that may benefit from MRI-guided DBS include Parkinson'sdisease, essential tremor, and multiple sclerosis. Parkinson's diseaseis a progressive neurological disorder in regions of the midbraincontaining a cluster of neurons known as the “substantia nigra.” Theseneurons produce the chemical dopamine, a neurotransmitter (messenger)responsible for transmitting signals between the substantia nigra andseveral clusters of neurons that comprise the basal ganglia and is vitalfor normal movement. When dopamine levels drop below 80%, symptoms ofParkinson's disease begin to emerge causing nerve cells of the basalganglia to fire out of control; resulting in tremor, muscle stiffness orrigidity, slowness of movement (bradykinesia) and loss of balance.Although medication masks some symptoms for a limited period, generallyfour to eight years in most patients, they begin causing dose-limitingside effects. Eventually the medications lose their effectiveness,leaving the patient unable to move, speak or swallow. Several preventiveand restorative strategies such as neural cell transplantation, neuralgrowth factors, gene therapy techniques and surgical therapies(including DBS), have shown promise in animal studies and human clinicaltrials. Important links to the cause (including genetic susceptibilityand the role of toxic agents) are becoming established. Leadingscientists describe Parkinson's as the neurological disorder most likelyto produce a breakthrough therapy and/or cure within this decade.Parkinson's disease afflicts approximately 1 million Americans, nearly40 percent of whom are under the age of 60. Roughly 60,000 cases of PDare diagnosed each year. It is estimated that Parkinson's disease costssociety $25 billion or more annually.

Essential tremor (ET) is considered the most common neurologicalmovement disorder affecting nearly 10 million people in the UnitedStates. ET is a chronic condition characterized by involuntary, rhythmictremor of a body part, most typically the hands and arms, often the headand voice, but rarely the legs. ET is generally considered a slowlyprogressive disorder, although many individuals may have a mild form ofET throughout life that never requires treatment. The most common formof ET affects the arms and hands, usually bilaterally, and is mostprominent with the arms held against gravity (postural tremor) or inaction (kinetic tremor) such as when writing or drinking from a cup.Unlike patients with Parkinson's disease, patients with ET rarelyexhibit a tremor when the arm is at rest. Pharmacological treatment forET includes a class of drugs called Beta-adrenergic blocking agents(such as propranolol), benefiting about 50 to 60 percent of patients.Primidone (MYSOLINE) is commonly regarded as the most effective drug.Side effects of these drugs include: bradycardia (slow heart rate),hypotension (low blood pressure), dizziness, fatigue, depression,diarrhea, nausea and/or sexual dysfunction. Surgical treatment of ET hasfor years involved placing a lesion in certain cluster of cells calledthe thalamus. This procedure, called stereotaxic thalamotomy has beenquite effective in substantially reducing tremor intensity, althoughthere is a finite risk of stroke or other surgical complications andbilateral thalamotomies increase the risk of speech impairment(dysarthria). The recent development of high frequency stimulation ofthe thalamus (deep brain stimulation) has provided a safer and moreeffective surgical strategy for treating ET. This procedure involves theplacement of an electrode in a region of the thalamus (VentralIntermediate Nucleus or VIM).

Multiple sclerosis (MS) tends to begin in young adulthood and affectsabout 500,000 people in the United States. Worldwide, the incidence rateis approximately 0.01% with Northern Europe and the northern US havingthe highest prevalence with more than 30 cases per 100,000 people. MS isa chronic, progressive, degenerative disorder that affects nerve fibersin the brain and spinal cord. A fatty substance (called myelin)surrounds and insulates nerve fibers and facilitates the conduction ofnerve impulse transmissions. MS is characterized by intermittent damageto myelin (called demyelination) caused by the destruction ofspecialized cells (oligodendrocytes) that form the substance.Demyelination causes scarring and hardening (sclerosis, plague) of nervefibers usually in the spinal cord, brain stem, and optic nerves, whichslows nerve impulses and results in weakness, numbness, pain, and visionloss. MS can affect any part of the central nervous system. When itaffects the cerebellum or the cerebellum's connections to other parts ofthe brain, severe tremor can result. Since the sub cortical gray matteralso contains myelinated nerve fibers, plaques can also be found in thestriatum, pallidum and thalamus. This may be the pathological basis forthe other movement disorders seen in a small proportion of patients withMS. Because different nerves are affected at different times, MSsymptoms often exacerbate (worsen), improve, and develop in differentareas of the body. Early symptoms of the disorder may include visionchanges (e.g., blurred vision, blind spots) and muscle weakness. MS canprogress steadily or cause acute attacks (exacerbations) followed bypartial or complete reduction in symptoms (remission). Most patientswith the disease have a normal lifespan.

In a typical DBS procedure, a stereotactic frame, e.g. an Ieksell frame,is attached (bolted) to the patient prior to any portion of the surgicalintervention. This is often done in a separate small operating room,either under sedation (Midazolam, Fentanyl, Propofol) and/or localanesthesia (Lidocaine). After the frame is attached, the patient istransferred to the table of the imaging system (CT or MR) and thepatient's head is immobilized. A box containing fiduciary markers isfitted on to the frame. These markers will show up in subsequent imagesin precisely known locations, allowing an accurate mapping between theframe coordinates and brain structures. Based on these detailed imagesand coordinate mappings, the trajectory for the surgery using a planningsoftware program.

Typical targets for the procedure include regions in the Thalamus, theGlobus Pallidum Internus (Gpi) and the Subthalamic Nucleus (SNT). Thetarget selection strongly depends on the disease and symptoms treated.DBS in the GPi seems to be very effective for drug-induced dyskinesiaand helps control tremor and bradykinesia. DBS in the SNT seem to bemost effective as measured by ability of patients to reduce theirmedications, however, there is a potential for increasing dyskinesia.The Thalamus is not necessarily a good target for patients withParkinson's disease but has been found to improve conditions forpatients with Essential Tremor and movement disorder caused by MultipleSclerosis.

Once the target has been effectively localized and noted to be in a safelocation, effort must be placed on a safe entry and trajectory to thetarget. MRI surface images of the cerebral cortex in combination withthe DBS planning scans can be useful to avoid injuries to cerebralarteries or veins at the initial drill holes and due to passage of theDBS electrode, resulting in a catastrophic hemorrhage. With thestereotactic software system, trajectory slices are possible so thatevery stage of the trajectory can be visualized in terms of itspotential harm as an electrode is passed toward the target. Fineadjustments to the entry point can be made to avoid these criticalstructures or avoid passage through the ventricular system in thepatient with large ventricles.

Entry point coordinates are not directly utilized during operativeplanning but are used by the computer system in creating the trajectoryitself. An estimate of accuracy can then be obtained and is usuallyaccurate within several hundred microns and always less than 0.5 cmaccuracy so that the results from imaging and planning can be usedeffectively during the surgical procedure.

Once the planning process is completed, the patient is transferred tothe operating room and a hole is drilled into the patient's skull (0.5″to 1.0″). At this point, most surgery centers will perform a real timephysiological localization of the target area to augment and verify thepreviously obtained stereotactic data by observing the patients responseto stimulation through the DBS electrodes or by recording and displaying(visual or audible) the action potentials of individual neurons alongthe path way to the target zone using microelectrodes. The additionalstep is considered necessary because the shape of the brain and theposition of anatomical structures can change during neuro-surgicalprocedures. Such changes can be due to differences between the patient'sposition in acquisition and during surgery, reduction in volume due totissue resection or cyst drainage, tissue displacement by theinstruments used, changes in blood and extra cellular fluid volumes, orloss of cerebrospinal fluid when the skull is opened. The amount ofbrain shift can in a severe case be a centimeter or more and is in mostcases between 1 and 2 mm.

In addition to the brain shift phenomenon, some subsection of specificnuclei cannot yet be identified by anatomic means, again requiring aphysiological determination of the target area. Given these“uncertainties,” several target runs may be required before the desiredresults are achieved. Throughout the procedure, responses from thepatient are necessary to determine if the target area has been reachedand if there are any unwanted site effects. Once the target area hasbeen correctly identified, the microelectrode is removed and replacedwith the DBS electrode. Stimulation voltage levels are determined byobserving the patient and the physiological response. Once allparameters have been correctly adjusted, the DBS electrode is anchoredin the skull, a pacemaker is implanted subcutaneously in thesubclavicular region and the lead is tunneled under the scalp up theback of the neck to the top of the head.

One of the major shortcomings with stereotactic DBS is the requirementof sub millimeter accuracy in electrode placement for the electricalstimulation of target areas deep inside the brain. As pointed out, brainshifts of 1 to 2 mm can routinely occur between the acquisition ofimages for the stereotactic surgery and the surgery itself and is eithercaused by patient transport (misregistration, image distortion), loss offluid (blood, CSF) or simple tissue displacement by the instrumentsused. A long recognized solution to these issues has been to performreal time MRI guided surgery. To this end a variety of MRI systems havebeen developed. “Open MRI” systems which are typically operated at fieldstrength ranging from 0.12 T (Odin) to 1.0 T (Philips) offer a clearadvantage in patient access over the closed bore systems ranging infield strength from 1.0 T to 3.0 T. However, these high field short boresystems outperform the low field systems in Signal-to-Noise Ratio sincethe SNR depends linearly on field strength. Higher SNR translatesdirectly into resolution and/or imaging speed. Efforts have beenundertaken to increase the field strength of these open systems (Philips1.0 T), however, it is not clear that much higher magnetic fields aredesirable or achievable due to considerable mechanical challenges ofstabilizing the separated pole faces of these magnets and the fact thatthese magnets are not easily shielded and have a larger fringe fieldthan comparable “closed bore” systems. Furthermore, significant progresshas been made to increase the patient access in high field systems aswell. Traditionally, whole body 3 T MRI systems have had a length inaccess of 2 m. Over the past few years dedicated head scanners (Allegra,Siemens) have been developed and have reduced the system length to 1.25m, allowing relatively easy access to the patient's head. Similarprogress has been made in whole body scanners at 1.5 T. Since the actualmagnet is significantly shorter (68 to 80 cm) than the overall systemfurther improvements in patient access can be expected. Image quality,speed and patient access are now at a point where true interventionalMRI is feasible. All major OEM's have recognized the need for a fullyintegrated MRI operating room and have made significant progress towardsthis goal. Siemens has introduced the “BrainSuite”, a fully integratedMRI suite for neuro-surgery. Philips, Siemens and GE have alsointroduced XMRI systems, combining 1.5 T or 3 T whole body systems withan X-Ray fluoroscopy with a patient table/carrier linking both systems.

Atrial fibrillation and ventricular tachyarrhythmias occurring inpatients with structurally abnormal hearts are of great concern incontemporary cardiology. They represent the most frequently encounteredtachycardias, account for the most morbidity and mortality, and, despitemuch progress, remain therapeutic challenges.

Atrial fibrillation affects a larger population than ventriculartachyarrhythmias, with a prevalence of approximately 0.5% in patients50-59 years old, increasing to 8.8% in patients in their 80's.Framingham data indicate that the age-adjusted prevalence has increasedsubstantially over the last 30 years, with over 2 million people in theUnited States affected. Atrial fibrillation usually accompaniesdisorders such as coronary heart disease, cardiomyopathies, and thepostoperative state, but occurs in the absence of any recognizedabnormality in 10% of cases. Although it may not carry the inherentlethality of a ventricular tachyarrhythmia, it does have a mortalitytwice that of control subjects. Symptoms which occur during atrialfibrillation result from the often rapid irregular heart rate and theloss of atrioventricular (AV) synchrony. These symptoms, side effects ofdrugs, and most importantly, thrombo-embolic complications in the brain(leading to approximately 75,000 strokes per year), make atrialfibrillation a formidable challenge.

Two strategies have been used for medically managing patients withatrial fibrillations. The first involves rate control andanticoagulation, and the second involves attempts to restore andmaintain sinus rhythm. The optimal approach is uncertain. In themajority of patients, attempts are made to restore sinus rhythm withelectrical or pharmacologic cardioversion. Current data suggestanticoagulation is needed for 3 to 4 weeks prior to and 2 to 4 weeksfollowing cardioversion to prevent embolization associated with thecardioversion. Chronic antiarrhythmic therapy may be indicated oncesinus rhythm is restored. Overall, pharmacologic, therapy is successfulin maintaining sinus rhythm in 30 to 50% of patients over one to twoyears of follow-up. A major disadvantage of antiarrhythmic therapy isthe induction of sustained, and sometimes lethal, arrhythmias(proarrhythmia) in up to 10% of patients.

If sinus rhythm cannot be maintained, several approaches are used tocontrol the ventricular response to atrial fibrillation. Pharmacologicagents which slow conduction through the AV node are first tried. Whenpharmacologic approaches to rate control fail, or result in significantside effects, ablation of the AV node, and placement of a permanentpacemaker may be considered. The substantial incidence of thromboembolicstrokes makes chronic anticoagulation important, but bleedingcomplications are not unusual, and anticoagulation cannot be used in allpatients.

In addition to medical management approaches, surgical therapy of atrialfibrillation has also been performed. The surgical-maze procedure,developed by Cox, is an approach for suppressing atrial fibrillationwhile maintaining atrial functions. This procedure involves creatingmultiple linear incisions in the left and night atria. These surgicalincisions create lines that block conduction and compartmentalize theatrium into distinct segments that remain in communication with thesinus node. By reducing the mass of atrial tissue in each segment, themass of atrial tissue is insufficient to sustain the multiple reentrantrotors, which are the basis for atrial fibrillation. Surgical approachesto the treatment of atrial fibrillation result in an efficacy of >95%and a low incidence of complications. However, despite these encouragingresults, this procedure has not gained widespread acceptance because ofthe long duration of recovery and risks associated with cardiac surgery.

Invasive studies of the electrical activities of the heart(electrophysiologic studies) have also been used in the diagnosis andtherapy of arrhythmias. Focal atrial tachycardias, AV-nodal reentranttachycardias, accessory pathways, atrial flutter, and idiopathicventricular tachycardia can be cured by selective destruction ofcritical electrical pathways with radiofrequency (RF) catheter ablation.Electrophysiologists have attempted to replicate the maze procedureusing RF catheter ablation. The procedure is arduous, requiring generalanesthesia and procedure durations often greater than 12 hours, withexposure to ionizing x-ray irradiation for over 2 hours. Some patientshave sustained cerebrovascular accidents. One of the main limitations ofthe procedure is the difficulty associated with creating and confirmingthe presence of continuous linear lesions in the atrium. If the linearlesions have gaps, then activation can pass through the gap and completea reentrant circuit, thereby sustaining atrial fibrillation or flutter.This difficulty contributes significantly to the long proceduredurations discussed above.

Creating and confirming continuous linear lesions and morbidity could befacilitated by improved minimally-invasive techniques for imaginglesions created in the atria. Such an imaging technique may allow theprocedure to be based purely on anatomic findings.

The major technology for guiding placement of a catheter is x-rayfluoroscopy. For electrophysiologic studies and ablation, frame rates of7-15 per second are generally used which allows an operator to seex-ray-derived shadows of the catheters inside the body. Since x-raystraverse the body from one side to the other, all of the structures thatare traversed by the x-ray beam contribute to the image. The image,therefore is a superposition of shadows from the entire thickness of thebody. Using one projection, therefore, it is only possible to know theposition of the catheter perpendicular to the direction of the beam. Inorder to gain information about the position of the catheter parallel tothe beam, it is necessary to use a second beam that is offset at someangle from the original beam, or to move the original beam to anotherangular position. The intracardiac electrogram may be used to guide thecatheters to the proper cardiac tissue.

Intracardiac ultrasound has been used to overcome deficiencies inidentifying soft tissue structures. With ultrasound it is possible todetermine exactly where the walls of the heart are with respect to acatheter and the ultrasound probe, but the ultrasound probe is mobile,so there can be doubt where the absolute position of the probe is withrespect to the heart.

Neither x-ray fluoroscopy nor intracardiac ultrasound have the abilityto accurately and reproducibly identify areas of the heart that havebeen ablated.

A system known as “non-fluoroscopic electro-anatomic mapping” (U.S. Pat.No. 5,391,199 to Ben-Haim), was developed to allow more accuratepositioning of catheters within the heart. That system uses weakmagnetic fields and a calibrated magnetic field detector to track thelocation of a catheter in 3D-space. The system can mark the position ofa catheter, but the system relies on having the heart not moving withrespect to a marker on the body. The system does not obviate the needfor initial placement using x-ray fluoroscopy, and cannot directly imageablated tissue.

Embodiments of fixed, steerable, cooled and Multi Electrode Array probesare described that may incorporate multiple functions, such as therecording of MRI imaging signals, bio potentials (electrophysiological,neurological) and cooling. The probes can significantly reduceheating-induced injury in materials surrounding them and can be easilyvisualized under MRI or X-ray. Disclosed embodiments are illustrativeand not meant to be limiting. Drawings illustrate exemplary embodimentsand design principles; absolute or relative dimensions are not to beinferred therefrom as necessarily pertaining to a particular embodiment.

FIG. 1 shows schematically an exemplary embodiment of a magneticresonance probe 100. The probe 100 may have a distal portion 7 and aproximal portion 8. The distal portion may include a plurality ofelectrodes, such as electrodes 3, 4, 5, 6. As shown, the electrodes maybe disposed at least partly on a surface of the probe 100. An electrodecan be disposed so that the electrode is disposed on the surface aroundthe circumference of the probe 100 (as shown for electrodes 4, 5, and6), disposed at the tip of the probe 100 (as shown for electrode 3), orso that the electrode is disposed at the surface around one or moreportions of the circumference. The probe 100 shown in FIG. 1 has fourelectrodes, but other numbers of electrodes may be provided, such as fewas one electrode. Probe 100 may include a plurality of centerconductors, such as center conductors 101, 102, 103, 104. Other numbersof center conductors may be provided. As shown in this exemplaryembodiment, center conductors 101, 102, 103, 104 may be coupled tocorresponding electrodes 3, 4, 5, 6. The center conductors may extendthrough the probe 100 and terminate in a connector 9 at the proximal endof the probe 100. One or more additional layers, described in greaterdetail below, may be disposed at least partially about the centerconductors in the proximal portion 8 of the probe 100.

FIGS. 2A-C depict additional features of an exemplary embodiment of aprobe 100. As shown in FIG. 2A, a junction J may define the transitionbetween the distal portion 7 and the proximal portion 8 of the probe100. The position of the junction J may be selected to provide the probe100 with preferred electrical properties, discussed in greater detailbelow. In an embodiment, the junction J may be positioned so that thedistal portion 7 of the probe 100 has a length approximately equal toone quarter the wavelength of an MR signal in the surrounding medium.For a medium such as blood or tissue, the preferred length for thedistal portion 7 can be in the range of about 3 cm to about 15 cm. Thecenter conductors 2 (referenced collectively) may be coiled to reducethe physical length of the distal portion 7 while maintaining the“quarter wave” electrical length. As shown in cross section FIG. 2B, thedistal portion 7 of probe 100 may include a plurality of centerconductors 2 and a lubricious coating 1 disposed the plurality of centerconductors. Exemplary lubricious coatings include polyvinylpyrrolidone,polyacrylic acid, hydrophilic substance, silicone, and combinations ofthese, among others.

With continued reference to FIGS. 2A and 2C, the proximal portion 8 ofthe probe 100 may include one or more additional layers disposed atleast partially about the plurality of center conductors 2. For example,a first dielectric layer 31 may be disposed at least partially about theplurality of center conductors 2. The first dielectric layer 31 maydefine a lumen 13 in which the plurality of center conductors 2 may bedisposed. An outer conductive layer 12 may be at least partiallydisposed about the first dielectric layer. The outer conductive layer 12may include a braiding. The outer conductive layer 12 may extend throughthe probe 100 and terminate at the connector 9. A second dielectriclayer 10 may be at least partially disposed about the outer conductivelayer 12. A lubricious coating 1 may be at least partially disposedabout the outer conductive layer 12 and/or the second dielectric layer10 in the proximal portion 8 of the probe 100.

As described above, a plurality of center conductors may be provided. Acenter conductor may include a conductive core. A center conductor mayinclude an insulator disposed at least partially about the core along atleast a portion of the core. The insulator may be disposed about thecore to prevent contact between various cores. The insulator may bedisposed along the entire length of the core or along one or portionsthereof. In an embodiment, an insulator may be disposed aboutsubstantially the entire length of a core except for a distal portionfor coupling to an electrode. Insulator may be selectively disposedabout core, such as discontinuously or on only a selected aspect of acore, such as an aspect that faces another core. Thus, insulator may bedisposed about one or more cores so that one or more center conductorsmay be touching but cores are not in contact.

The insulator can facilitate positioning a center conductors in closeproximity to another center conductor. For example, two centerconductors may touch but not have the respective cores be in contact.Such close arrangement of center conductors can permit electricalcoupling between the center conductors of high-frequency energy, such asmagnetic resonance energy, while preventing coupling of low-frequencyenergy between the center conductors. Coupling the center conductors forhigh-frequency energy facilitates receiving magnetic resonance signalswith the center conductors because the center conductors so coupled canact as a single electrical entity with respect to the high-frequencyenergy. Thus, the electrical length of the distal portion 7 of the probe100 can be preserved, because magnetic resonance energy can be conductedstraight through the plurality of center conductors, without allowingthe magnetic resonance energy to pass separately through variousconductors, thereby creating interference, or causing the high-frequencyenergy to move through a longer path, thereby unbalancing a magneticresonance antenna. In contrast, a thin insulating layer can besufficient to prevent coupling between conductors of the low-frequencysignals that may be conducted along selected center conductors. Forexample, low-frequency coupling may not be desirable when the probe 100is being operated to measure an electrical potential between twoelectrodes contacting various tissue regions. If the center conductorswere permitted to couple this low-frequency energy, then the potentialmeasurement could be distorted, lost in excessive noise, or attenuatedentirely. Similarly, ablation energy delivered along the probe 100 couldbe shorted between center conductors if the center conductors werepermitted to couple low frequency energy.

Thus, the wire insulation is preferably sufficiently thin so that thecenter conductors are electrically coupled through the insulator at highfrequency (e.g., above 10 MHz) but are isolated at frequencies below 0.5MHz.

Accordingly, insulator properties may be selected to facilitate couplingof high-frequency energy between center conductors, while lessening orinhibiting coupling of low-frequency energy. Properties include thematerial or materials from which the insulator is made, the thickness ofthe insulator, the number of layers of insulator, the strength of themagnetic field in which the probe 100 may be immersed, among others.

Because the insulator can prevent coupling of low-frequency energybetween the center conductors, the center conductors can be brought intovery close proximity to one another, also termed “tightly coupled” toone another. The center conductors may be tightly coupled, for example,by twisted around one another. Twisting or otherwise tight-coupling thecenter conductors facilitates keeping the center conductors in closeproximity in the distal portion 7 of the probe 100, where there may beno, e.g., first dielectric layer to keep the center conductors closelyapposed. In addition, because reactive elements need not be interposedbetween the center conductors to decouple low-frequency energy,manufacture of the probe is simplified. Furthermore, the absence ofreactive elements can permit the achievement of small probe diameters.For example, a probe having an outer of diameter of about 15 French orless, suitable for, among other uses, cardiac catheterization,observation, and/or ablation, can be readily constructed using systemsand methods disclosed herein. Moreover, deep brain stimulation with amagnetic resonance probe is facilitated, because the diameter can bereduced to, for example, 4 French or less, 3 French or less, 2 French orless, 1.3 French or less, 1 French or less, 0.5 French or less, or even0.1 French or less. The outer diameter can be affected by the thicknessof the center conductor core, thickness of insulator, and thicknesses ofother layers that may be included. In an embodiment, wire may be usedhaving a thickness of 56 AWG to 16 AWG as well as thinner and/or thickerwire.

A preferred insulator thickness may be determined as follows. Theinductance L and capacitance C between a twisted pair of wires per unitlength is given by the equations:

$\begin{matrix}{L = {( \frac{\mu_{0}}{\pi} )\cos\;{h^{- 1}( \frac{D}{d} )}}} \\{C = {C_{1} + C_{2} - C_{3}}} \\{C_{1} = {\int_{a}^{b}\ \frac{ɛ_{0}{\mathbb{d}x}}{D + {( {{1.0/ɛ_{r}} - 1.0} )\sqrt{D^{2} - x^{2}}} - \frac{\sqrt{d^{2} - x^{2}}}{ɛ_{r}}}}} \\{C_{2} = \frac{{\pi ɛ}_{0}}{\cos\;{h^{- 1}( \frac{D}{d} )}}} \\{C_{3} = {\int_{a}^{b}\frac{ɛ_{0}{\mathbb{d}x}}{D - \sqrt{d^{2} - x^{2}}}}}\end{matrix}$where ε₀=8.854 pF/m, d is the bare wire diameter in meters, D is theinsulated wire diameter in meters, and ε_(r) is the relative dielectricconstant of the insulating material. In one illustrative embodiment, a33 AWG magnet wire was used, the wire having a nominal bare wirediameter of 0.0071″ (0.00018034 m) and an insulated diameter of 0.0078″(0.00019812 m) and an approximate dielectric constant of ε_(r)=2. Thus,the insulator thickness was about 17.78 microns, or about 8.89 micronson a side. In this exemplary case the estimated capacitance per unitlength is 89 pF/m. This corresponds to a capacitive impedanceZ_(c)=1/(2*π*f) of about 28 Ω/m at 63.86 MHz and giving a good couplingat the high frequency range. Because the impedance scales inversely withfrequency, the low frequency impedance at 100 kHz is estimated to be 14kΩ/m. An impedance of 10 kΩ/m or greater is sufficient in mostapplications to provide sufficient decoupling. The high frequencyimpedance is preferably kept below 100 Ω/m.

The impedance can also be controlled by the choice of dielectricmaterial. Typical materials include polyurethane resins, polyvinylacetal resins, polyurethane resins with a polyimide (nylon) overcoat,THEIC modified polyester, THEIC modified polyester with a polyamideimide(AI) overcoat, THEIC modified polyester, oxide-based shield coat and apolyamideimide (AI) overcoat, aromatic polyimide resin, bondablethermoplastic phenoxy overcoat, glass fiber, All Wood Insulating CrepePaper, Thermally Upgraded Electrical Grade Crepe Kraft Paper, HighTemperature Aramid Insulating Paper, and combinations of these. Thelength of the proximal portion can be modified by selecting dielectricmaterials for the first dielectric layer and/or second dielectriclayers. For example, a material with a high dielectric constant can beincorporated in one or more dielectric layers, thereby decreasing theelectrical length of the proximal portion and facilitating use of aprobe in a relatively shallow anatomic location. Examples of materialswith appropriate dielectric constants include ceramics.

An insulator disposed at least partially about a center conductor coremay have a thickness in a range up to about 2,000 microns, preferably upto about 500 microns, more preferably up to about 200 microns, stillmore preferably up to about 100 microns, yet more preferably in a rangebetween about 1 micron and about 100 microns. An insulator may have athickness in the range of about 5 microns to about 80 microns. Aninsulator may a thickness in the range of about 8 microns to about 25microns. An insulator may a thickness in the range of about 10 micronsto about 20 microns.

A core may have an insulator disposed about it by dipping the core ininsulator. A core may have an insulator disposed about it by extrudingan insulator over the core. A core may have an insulator disposed aboutit by sliding the core into an insulator or sliding an insulator over acore. A core may have an insulator disposed about it by spraying.

A core may be formed of wire. The wire is preferably thin, to promotesmall probe size, and may in one embodiment be thin insulated copperwires (33 AWG), at times silver coated. In preferred embodiments, thecenter conductors are formed of magnetic-resonance compatible material.Preferably, the materials are highly conducting, such as silver cladcopper. The outer conductive layer may also be formed of wire, such asbraided wire. Other preferred materials include a super elasticmaterial, copper, gold, silver, platinum, iridium, MP35N, tantalum,titanium, Nitinol, L605, gold-platinum-iridium, gold-copper-iridium, andgold-platinum.

As mentioned previously, the plurality of center conductors 2 in thedistal portion 7 of the probe 100 may form a first pole of a dipole(loopless) magnetic resonance antenna, while the outer conductive layer12 in the proximal portion 8 of the probe 100 can form the second pole.As discussed above, the length of the distal portion, or first pole, ispreferably approximately the “quarter-wave” length, typically about 3 cmto about 15 cm. The proximal portion or second pole can be of the samelength, so that the dipole antenna is balanced. A balanced dipoleantenna can provide slightly improved signal quality compared to anunbalanced dipole antenna. However, a proximal portion of approximatelyeven 15 cm may be impractical, because a user might want to introduce amagnetic resonance probe into body structures deeper than 15 cm. Inpractice, it has been found, fortuitously, that lengthening the proximalportion or second pole, while unbalancing the antenna and slightlydegrading image quality, permits visualization of a substantial lengthof the antenna, which facilitates tracking and localization of theantenna. A significant complication of unbalancing the antenna, namelyheating effects during the transmission mode, can be avoided bydecoupling the antenna with, for example, a PIN diode, as describedbelow. FIGS. 7A-B depict the effects of decoupling an unbalancedantenna. FIG. 7A shows a heating profile of a decoupled antenna, whichcauses minimal heating to surrounding tissue (typically less than 0.5degrees Celsius), while FIG. 7B shows a heating profile of anon-decoupled antenna, which can cause gravely injurious and possiblefatal tissue heating of over 20 degrees Celsius in a matter of seconds.Adjustments can typically be made to matching, tuning, and/or decouplingcircuits, examples of which are shown in FIGS. 3A-E.

The circuits shown in FIGS. 3A-E may have multiple functions and canbest described by examining four particular situations, the transmitphase of the MRI system, the receive phase of the MRI system, therecording of electrophysiological signals and the stimulation or deliverof energy of or to the organ or tissue of interest.

The MRI system typically alternates between a transmit and receive stateduring the acquisition of an image. During the transmit phase relativelylarge amounts of RF energy at the operating frequency of the system,such as about 63.86 MHz, are transmitted into the body. This energycould potentially harm the sensitive receiver electronics and moreimportantly, the patient, if the imaging antenna, in this case theprobe, would be allowed to pick up this RF energy. The antenna functionof the probe therefore is preferably turned off so that the probebecomes incapable of receiving RF energy at the MRI system operatingfrequency. During the receive phase, in contrast, the body emits the RFenergy absorbed during the transmit phase at the same frequency, i.e.,63.86 MHz. A significant amount of the transmitted energy is typicallylost due to inefficiencies of the transmitter or has been converted intoheat by the body. The RF signal emitted by the body containing the imageinformation is typically therefore many orders of magnitude smaller thanthe original signal send out by the transmitter. In order to receivethis small signal, the antenna function of the probe is preferablyturned on so that the probe becomes a highly efficient receiver for RFsignals at the MRI systems operating frequency. The alternating state ofthe probe from being a poor RF antenna (receiver) during the transmitphase to being a good RF antenna (receiver) during the receive phase iscalled T/R (Transmit/Receive) switching and may be facilitated via acontrol signal send by the MRI system on the center conductor ofconnector 15 in FIG. 3A. In an embodiment, this signal may be a smallpositive voltage (5 to 15 Volts) during the transmit phase, and a smallnegative voltage (−5 to −20 Volts) during the receive phase. During theimage acquisition, the system typically alternates between the transmitand receive phase within milliseconds, i.e., at about a kHz frequency.

During the transmit phase, the positive voltage on the center conductorof connector 15 with respect to the system ground 14 may cause the PINdiode 21 to be conductive and can therefore short the top end ofcapacitors 23 to ground. The capacitors 23 in combination with theproximal length of the probe form a transmission line; thus, theimpedance at the top of the capacitor 23 can be transformed via thistransmission line to an impedance Z_(J) at the junction J connecting thepoles of the electric dipole antenna in FIG. 2A. A high impedance atthis junction is preferable to disable the reception of RF energy. Toachieve a high impedance at the junction J with shorted capacitors 23,the transmission line should have an electrical length equivalent to aquarter wavelength for RF propagation inside the transmission line. Thecapacitance values for capacitors 23 may be selected to fine-tune theeffective electrical length of the transmission line using routineexperimentation. Typical values for capacitors can fall in the range of1-10,000 pF. The precise values of individual capacitors 23 may varyslightly because each center conductor may have a slightly differentlength (because center conductors may be coupled to electrodes disposedat various positions along the probe). In an embodiment, high Qcapacitors such as ATC 100 A or B are preferred. The wavelength may bedetermined by the diameter of the center conductor bundle, thedielectric constant of the dielectric material, and the inner diameterof the outer conductive layer. In a typical exemplary embodiment, thephysical length of the proximal section of the probe forming thetransmission line may be 90 cm. Disabling the antenna function of theprobe by presenting a high impedance at the junction J is known as“decoupling.”

With continued reference to FIGS. 3A-E, during the receive phase, anegative voltage on the center conductor of connector 15 with respect tothe system ground 14 can “reverse bias” the diode 21, thereby renderingit non-conductive. The antenna impedance seen by the MRI system ispreferably near 50 Ω for optimal performance. Typically, the impedanceof the electric dipole antenna and the capacitors 23 is transformed topresent the appropriate impedance to the systems. This transformationmay be achieved via selection of appropriate inductor 19 and capacitor17. Preferably, the values for elements 19 and 17 may be chosen to passlow frequency current, such as a switched DC signal to diode 21.

The T/R switching voltages are preferably not passed onto the probesince the switching voltage, which can have a frequency around 1 kHz,may cause unwanted stimulation of the organ or tissue under examination.To combat this, capacitors 23, providing a high-pass filter function,can block propagation of the T/R switching voltage into the probe.

With further reference to FIGS. 3A-E, because the antenna function ofthe probe is enabled during the receive phase, the antenna will pick upRF (63.86 MHz) signals emitted from the body. As shown in FIG. 3A, theRF signal may be routed through the capacitors 23 to the MRI systemconnector 15 and is processed by the MRI system. As described above,capacitors 23 may function as high-pass filters so that thehigh-frequency MRI signal is passed to the MRI system, but lowerfrequency signal, such as the switching signal, electrophysiologicalstimulation signal, biopotential measuring signal, and/or ablativeenergy signal are blocked. The lower-frequency signals may instead berouted through another circuit, depicted in FIG. 3B. The signal atcontacts 24 may be split into two sets of leads, one set conveying thehigh-frequency magnetic resonance signal to the magnetic resonancesignal pathway that may include capacitors 23 (FIGS. 3A and C), and theother set conveying lower frequency signals to the electrophysiologypathway that may include inductors 22 (FIGS. 3B and 3D). The inductors22 can be chosen to block the high-frequency MRI signal (typicallyaround 64 MHz for a 1.5 Tesla field strength) but to pass lowerfrequency signals such as the electrophysiological signals from thebrain, the heart, etc. Capacitors 20 can be provided to shunt to groundMRI signal “leaking” through inductors 22. Thus, inductors 22 andcapacitors 20 may form a low pass filter. Exemplary values to filterhigh frequency MR signal at about 63.86 MHz can be about 10,000 pF forcapacitors 20 and 5.6 μH for inductors 22.

Electrophysiological (EP) signals may be measured independently of theTransmit/Receive state of the MRI system because these signals aretypically in a frequency range far below the MRI signal frequency andare separate from the MRI signal via a filter, such as the signal splitand low-pass filter depicted in FIG. 3B and effected by inductors 22 andcapacitors 20. The EP signals may pass through this low pass filter tothe connector 16 and can be routed to the EP recording system, tissuestimulator, ablation energy source, or the like. Similarly, tissuestimulation and/or tissue ablation can be done independently of theTransmit/Receive state of the MRI system because energy sent through theconnector 16 from either an ablation energy source, a cardiacstimulator, a neurostimulator, etc. is at sufficiently low enoughfrequencies, typically less than 500 kHz, that it will pass through thelow pass filter network shown in FIG. 3B and be conveyed into the probeto one or more electrodes 3, 4, 5, 6, but will be blocked from enteringthe MRI system by the high pass filter formed by capacitors 23 in FIG.3A. Examples of low voltage signals include those for the treatment ofParkinson's disease as part of Deep brain stimulation and RF energy atseveral hundred kilohertz that may cause, among other effects, ablationof heart tissue. In the latter case, the stimulus may be provided toonly one electrode, e.g., electrode 3, which may be located at the tipof the probe, to facilitate precise delivery of heat therapy and toprovide in some embodiments a large contact area.

As depicted in FIGS. 3C-E, the magnetic resonance pathway can bedisposed on one substrate 26, and the electrophysiology pathway can bedisposed on another substrate 28. The substrates may be coupled to aground plane 29. The signal split at contacts 24 may be provided throughholes in substrate 26 to permit a connection to contacts 27 for theelectrophysiology pathway.

With further reference to FIGS. 2A-C and 3A-E, contacts 24 can mate withthe appropriate pins in the connector 9. The outer conductive layerconnector in connector 9 (ground) can mate with ground pin 25. Duringthe transmit phase of the MR system, the pin diode 21 can be activated,as described above, and can thereby create a short between the pluralityof center conductors 2 and the outer conductive layer 12. As describedabove, the electrical length of the outer conductive layer 12 andcapacitors 23 may be chosen so that the short at diode 21 transfers downthe transmission line into an open at junction J at which the outerconductive layer terminates.

FIGS. 4A-C depict an embodiment in which a probe is constructed to besteerable (bi-directional). Many of the features are as discussed forthe embodiments shown in FIGS. 1 and 2A-C. The probe 100 may include aribbon 36 disposed in the distal portion 7 of the probe 100. In anembodiment, the ribbon 36 can extend to the tip of the probe 100. Theribbon 36 can be bonded to the tip. The probe 100 can further include apull wire 46. The pull wire 46 can be coupled to the ribbon 36 so thatthe ribbon 36 may flex when the pull wire 46 is manipulated. The pullwire 46 may be disposed in a lumen 30 in the probe 30. The pull wire 46may be coupled to a, for example, a steering disc 33, which may bedisposed in a handle 34 for the user's convenience. The plurality ofcenter conductors 2 may be radially centered; they may be offset; theymay be disposed in a multi-lumen polymeric tubing; they may run alongthe length of the probe. A second and/or additional lumens 30 can beprovided. A second and/or additional pull wires 46 can be provided. Inthe distal portion 7, the steering assembly may be housed in a thinwalled flexible polymeric tubing to prevent direct electrical contactwith the center conductors 2 and/or electrodes 3, 4, 5, 6. In the distalsection the conductors may or may not be centered, may be straight orcoiled (around the steering mechanism assembly), and/or may be connectedto the electrodes electrically. The steering mechanism if modified intoa loop coil can have a different matching-tuning and a decouplingcircuit. The matching tuning and decoupling circuitry for a steeringmechanism acting as a loopless antenna can be combined with that of theconductors connecting to the electrodes. Materials used for the pullwires 46 may include non-metallic materials e.g. carbon fiber,composites, nylon, etc to prevent the pull wires interacting with thecenter conductors 2. The pull wires 46 can also be made from conductingmaterials and turned either into loop or loopless coils based describedelsewhere.

FIGS. 5A-C depict a similar embodiment to the one shown in FIGS. 4A-C,with a coolant lumen 38 that may be provided to allow the flow ofcoolants. Exemplary coolants include saline solution, cooled gases, suchas nitrogen, and water, among others.

Probes disclosed herein can facilitate three dimensionalelectro-anatomical imaging. As depicted in FIGS. 6A-D, a probe can bemodified to a multi electrode array probe. The multi electrode arrays(MEA) can be arranged on an expandable basket type probe. This MEA probecan be used, for example, for non-contact or contact endocardialmapping. The probe 100 may include a plurality of expandable arms. Theprobe 100 may include a first dielectric layer 43. The probe 100 mayinclude an outer conductive layer 42. The probe 100 may include a seconddielectric layer 41. The probe 100 may include a shaft 44 to push thebasket and expand it. The probe 100 may include a bundle 45 of 8insulated tightly coupled conductors, resembling the center conductorsdescribed above, but in this embodiment with more conductors in thebundle and multiple bundles.

An electrode can be disposed on an arm. An electrode may be affixed toan arm. An electrode may be glued or bonded to an arm. An arm mayinclude more than one electrode. A basket probe with, e.g., 8 expandableribs and each carrying, e.g., 8 electrodes is depicted. FIG. 6B depictsa long-axis view of an expandable arm 39, showing 8 electrodes disposedon the arm. During insertion into the body the basket array probe may becollapsed to form a low profile probe, once inside the desired anatomicspace to be mapped, such as a cardiac chamber, the basket may beexpanded. The basket may be expanded, for example, by coupling a pullwire to one or more arms, or by forcing expansion with hydraulic force.The basket can expand to a variety of sizes, such as space-limited bycontacting the walls of the anatomic site, or to a fixed diameter,dimension, and/or shape, such that the arms of the basket expand in acontrolled manner, e.g. a cylinder. Mapping may then be carried out bynon-contact mapping. The electrical potentials measured at theelectrodes may be translated to the potentials on the endocardium. Thearms can be formed of materials similar to those used for centerconductors, as described above. The basket can be opened and closed byadvancing and retracting a sliding inner tubing. The proximal shaft mayinclude a sliding tubing centered in the outer assembly which houses theconductors, dielectric/insulator, shielding and an outer tubing. Thisassembly can act like a loopless antenna, the shielding/braiding in theproximal shaft acts as the ground, and the conductors connecting to theindividual electrodes act as the whip of the antenna. This assembly canbe matched-tuned and/or decoupled using systems and methods describedabove. The probe can be provided with a curved tip for, e.g.,maneuvering. An ablation electrode can be incorporated as describedabove, such as at the distal tip. Steering systems as described abovecan be provided. A steerable ablation multielectrode array canfacilitate mapping and treating tissue simultaneously. In an embodiment,a non-contact EP map can be superimposed on a 3-D MR image of theendocardium by using techniques described in, e.g., U.S. Pat. No.5,662,108, hereby incorporated herein by reference. In an embodiment,miniature loop coils may be placed adjacent one or more electrodes totrack the position of the one or more electrodes and the distance fromthe electrode to the tissue wall.

FIGS. 8A-C depict schematic diagrams of an exemplary embodiment of abi-directional steerable probe. In an embodiment, a steerable probe mayhave two sections, a stiff proximal section and a steerable distalsection. In an embodiment, a steerable distal section can have a lengthin the range of about 1 cm to about 15 cm. The steering can be achievedby including a fixed ribbon wire in the distal section of the probe. Theproximal section of the flat ribbon wire can be anchored in thetransition between the stiff and flexible sections. The transition mayinclude a joint, such as a weld or a spot adhesive. The distal end ofthe flat wire can be bonded to the distal tip of the probe. The pullwires/steering mechanism wires may run along the length of the probe.The proximal end of the pull wires can be attached to the steeringmechanism. The distal end of the pull wires/steering mechanism may beattached to the distal end of the flat ribbon, which is then bonded tothe distal tip of the probe. In operation, pulling or releasing the pullwire can bend or steer the distal tip in the direction of the pull. Theextent of the bending typically depends on at least one of the innerdiameter (ID) of the outer tubing (distal section), the overallstiffness of the tubing/assembly, and on other properties of theassembly. Steerable probes may be modified so that they work like a RFloop antenna coil, so that they may be actively tracked under MR. Thishelps the operating clinician to know the exact position of the probe inthe anatomy.

Steerable probes may be modified for MR compatibility by usingnon-magnetic materials. Steerable probes may be modified for MRcompatibility by using materials which create few or no susceptibilityartifacts. Appropriate materials include, e.g., polymers/plastics,metals—Nitinol, copper, silver or gold, gold platinum alloy, MP35Nalloy, etc. An exemplary design of the probes is shown in FIGS. 8A-C.The proximal shaft of the probe may include a multi-lumen tubing with atleast 2 lumens parallel to each other. These lumens can house a numberof pull wires, such as 2 pull wires. The pull wires may be connected tothe steering handle at the proximal end, and at the distal end they maybe connected to the distal end of the flat ribbon wire assembly, theproximal end of which may be anchored in the transition. The twoparallel pull wires connected to the flat steering ribbon at the distalend can form a loop antenna which can then be matched-tuned and/ordecoupled by the circuitry in the proximal handle. This creates an MRcompatible, MR safe bi-directional steering probe whose position can betracked under MRI.

Alternatively, as shown in FIGS. 8A-C, a bi-directional steerable probemay include a loopless antenna. In this exemplary embodiment, the outerproximal tubing has a braid under it or in the wall of the outer tubing.This assembly acts like a loopless antenna, with the pull wires and theflat ribbon assembly as the whip and the braiding in or under the outertubing as the ground forming a loopless antenna. The matching-tuning anddecoupling circuits may be built proximal to the probe, e.g. in thesteering handle. This design enables the probe to be tracked under MRand capable of acquiring high resolution images in the vicinity of theprobe.

FIGS. 9A-C and 10A-C depict exemplary embodiments of unidirectionalsteerable probes. These embodiments may be similar to in design to theloopless bi-directional steerable probe, except that there is a singlepull wire. This design can be used to image under MRI and also to betracked under MR. The proximal shaft/section can have a braiding in thewall or under the outer tubing. The pull wire may run radially in thecenter of the tubing thus creating a structure similar to a coaxialcable (FIGS. 10A-C) or can be radially offset from the center (FIGS.9A-C). The matching-tuning and decoupling circuit can be built in theproximal section of the probe, making it function similar to a looplessantenna, and/or enabling it to be tracked under MRI. It can also be usedto acquire high-resolution images of the anatomy around the probe.

Additional teachings regarding construction of magnetic resonanceprobes, selection of materials, preferable dimensions of components, andelectrical properties of probes are provided, e.g., in U.S. Pat. Nos.5,928,145, 6,263,229, 6,549,800, and in U.S. patent applicationPublication Ser. Nos. US 2002/0,045,816 A1, US 2002/0,161,421 A1, US2003/0,028,095 A1, and US 2003/0,050,557 A1, all of which patents andpatent application publications are hereby incorporated herein in theirentireties by this reference.

While the disclosed systems and methods have been described inconnection with embodiments shown and described in detail, variousmodifications and improvements thereon will become readily apparent tothose skilled in the art. Accordingly, the spirit and scope of thepresent disclosure is limited only by the following claims.

We claim:
 1. A magnetic resonance probe, comprising: a plurality ofcenter conductors, at least some center conductors; including aconductive core and an insulator disposed at least partially about thecore along at least a portion of the core; and forming a first pole of amagnetic resonance dipole antenna; a first dielectric layer disposed atleast partially about the plurality of center conductors in a proximalportion of the probe; an outer conductive layer at least partiallydisposed about the first dielectric layer and forming a second pole ofthe magnetic resonance dipole antenna; and a plurality of electrodes, atleast one electrode being coupled to one of the center conductors anddisposed at least partly on a probe surface.
 2. The probe of claim 1,further comprising a second dielectric layer at least partially disposedabout the outer conductive layer.
 3. The probe of claim 1, furthercomprising a lubricious coating at least partially disposed about theouter conductive layer.
 4. The probe of claim 1, wherein the pluralityof center conductors are magnetic resonance-compatible.
 5. The probe ofclaim 1, wherein at least one insulator has a thickness equal to or lessthan about 100 microns.
 6. The probe of claim 1, wherein at least somecenter conductors comprise at least one of a magnetic resonancecompatible material, a super elastic material, copper, silver-copper,gold, silver, platinum, iridium, MP35N, tantalum, titanium, Nitinol,L605, gold-platinum-iridium, gold-copper-iridium, and gold-platinum. 7.The probe of claim 1, further comprising a connection to a high-passfilter through which the probe is coupleable to a magnetic resonancescanner.
 8. The probe of claim 1, further comprising a connection to alow-pass filter through which the probe is coupleable to at least one ofan electrophysiological recording system, a tissue stimulator, and anablation energy source.
 9. The probe of claim 1, further comprising: aribbon disposed in a distal portion of the probe; and a pull wirecoupled to the ribbon.
 10. The probe of claim 9, wherein the pull wireis disposed in a lumen in the probe.
 11. The probe of claim 1, furthercomprising a coolant lumen.
 12. The probe of claim 1, further comprisinga plurality of radially expandable arms, wherein at least one electrodeis at least partly disposed on one arm.
 13. The probe of claim 12,further comprising a tubing that is slideably displaceable between atleast two positions to transition the expandable arms between aretracted position and an expanded position.
 14. The probe of claim 12,further comprising a tubing that is slideably displaceable between atleast two positions to transition the expandable arms between aretracted position and an expanded position.
 15. The probe of claim 1,further comprising an ablation electrode disposed at a distal tip of theprobe.
 16. The probe of claim 1, further comprising an interface circuitcoupled to the probe, the interface circuit including: a signal splitterthat directs a signal received from the probe to a magnetic resonancepathway and an electrophysiology pathway; a high-pass filter disposed inthe magnetic resonance pathway; a low-pass filter disposed in theelectrophysiology pathway; a connector disposed in the magneticresonance pathway for connecting to a magnetic resonance scanner; and aconnector disposed in the electrophysiology pathway for connecting to atleast one of a tissue stimulator, a biopotential recording system, andan ablation energy source.
 17. The probe of claim 1, wherein the probehas an outer diameter of less than about 15 French.
 18. The probe ofclaim 1, wherein the probe has an outer diameter of less than about 4French.
 19. The probe of claim 1, further comprising a connector portiondisposed at a proximal end of the probe, the connector portionincluding: an outer conductor contact coupled to the outer conductivelayer; extended sections of at least some center conductors extendingproximally beyond the outer conductor contact, at least one extendedsection having a center conductor contact coupled to one centerconductor; and an insulated area interposed between the outer conductorcontact and the at least one center conductor contact.
 20. The probe ofclaim 1, defining at least one lumen.
 21. The probe of claim 20, furthercomprising a pull wire disposed in the lumen, coupled to a distalportion of the probe, and longitudinally displaceable.
 22. The probe ofclaim 1, wherein at least one center conductor is coupled to a distalportion of the probe and longitudinally displaceable.
 23. The probe ofclaim 1, wherein all of the center conductors collectively form thefirst pole of the magnetic resonance dipole antenna.
 24. A method ofperforming a magnetic resonance-guided procedure, comprising: placing asubject in a magnetic resonance scanner; identifying a target site inthe subject using data about the subject obtained from the scanner;introducing into the patient a magnetic resonance probe as defined byclaim 1; advancing the probe to the target site; and performing theprocedure using the magnetic resonance probe.
 25. The method of claim24, wherein the target site is located in the subject's brain, and theprobe is introduced by employing a stereotactic frame.
 26. The method ofclaim 24, wherein the target site comprises at least one of thesubject's thalamus, globus pallidum internus, and subthalamic nucleus.27. The method of claim 24, further comprising anchoring at least one ofthe probe's electrodes in the subject.
 28. The method of claim 24,further comprising electrically connecting at least one of the probe'selectrodes to a pacemaker.
 29. The method of claim 24, wherein thetarget site is located in the subject's heart.
 30. The method of claim29, wherein at least one probe electrode is an RF ablation electrode,and the method further comprises ablating heart tissue.
 31. The methodof claim 30, wherein ablating comprises creating a plurality of linearablations in the subject's left and/or right atrium.
 32. A combinedmagnetic resonance imaging and electrophysiology probe, comprising: aplurality of center conductors, at least some center conductorsincluding a conductive core and an insulator disposed at least partiallyabout the core along at least a portion of the core, the insulatorhaving a thickness equal to or less than about 100 microns; a firstdielectric layer disposed at least partially about the plurality ofcenter conductors in a proximal portion of the probe; an outerconductive layer at least partially disposed about the first dielectriclayer; a second dielectric layer disposed at least partially about theouter conductive layer; and a plurality of electrodes, at least oneelectrode coupled to one of the center conductors and disposed at leastpartly on the probe surface.
 33. A system for magnetic resonanceimaging, comprising: a magnetic resonance probe, including: a pluralityof center conductors, at least some center conductors: including aconductive core and an insulator disposed at least partially about thecore along at least a portion of the core; and forming a first pole of amagnetic resonance dipole antenna; a first dielectric layer disposed atleast partially about the plurality of center conductors in a proximalportion of the probe; an outer conductive layer disposed at leastpartially about the first dielectric layer and forming a second pole ofthe magnetic resonance dipole antenna; and a plurality of electrodes, atleast one electrode coupled to one of the center conductors and disposedat least partly on the probe surface; and a interface electricallycoupled to the probe, the interface including: a signal splitter thatdirects a signal received from the probe to a magnetic resonance pathwayand an electrophysiology pathway; a high-pass filter disposed in themagnetic resonance pathway; a low-pass filter disposed in theelectrophysiology pathway; a connector disposed in the magneticresonance pathway for connecting to a magnetic resonance scanner; and aconnector disposed in the electrophysiology pathway for connecting to atleast one of a tissue stimulator, a electrophysiological recordingsystem, and an ablation energy source.
 34. A magnetic resonanceinterventional probe assembly, comprising: a magnetic resonance probe,comprising a steerable distal section attached to a stiff proximalsection with a plurality of electrodes held on the distal section, thedistal section including a plurality of center conductors which form afirst pole of a magnetic resonance dipole antenna and an outerconductive layer which forms a second pole of the antenna, each centerconductor including a conductive core and an insulator disposed at leastpartially about the core, the insulator being configured to electricallycouple the plurality of center conductors together when exposed to highfrequency radiation and electrically decouple the plurality of centerconductors from each other when exposed to low frequency radiation; andat least one pull wire connected to a distal tip portion of the probe toallow the distal tip to flexibly bend in response to pulling of the pullwire, wherein the probe cooperates with the at least one pull wire todefine a loopless magnetic resonance antenna which allows MRIvisualization of the probe tip.
 35. A magnetic resonance interventionalprobe assembly according to claim 34, wherein the plurality of insulatedcenter conductors are held in a lumen defined by a first dielectricmaterial.
 36. A magnetic resonance interventional probe assemblyaccording to claim 34, further comprising a decoupling circuitcomprising a PIN diode, wherein during a receive phase of an MRIscanner, a negative voltage can reverse bias the diode thereby renderingthe diode non-conductive, and wherein during a transmit phase of the MRIscanner, a positive voltage cause the PIN diode to be conductive.